The invention relates to a method for processing pulses supplied by a gamma camera and to a gamma camera applying this method. It relates to scintillation or gamma cameras, of the ANGER type of which the fundamental operation and embodiments are disclosed in U.S. Pat. No. 3,011,057. These gamma cameras are intended for detection and visual display of the photons emitted by radioactive substances.
Gamma cameras are utilised in nuclear medicine to provide a visual display of the distribution within an organ of molecules tagged by a radioactive isotope which had been injected into a patient. A gamma camera commonly comprises a collimator for focussing the gamma photons emitted by the patient, a scintillator crystal for converting the gamma photons into luminous photons or scintillations, and a grid of photomultiplier tubes which each converts the scintillations into electrical pulses referred to as electrical tube contributions. It also comprises electronic circuits arranged to derive from the electrical contributions supplied by the photomultiplier tubes, signals giving the X and Y coordinates of the location at which the scintillation had occurred, as well as a validation signal Z when the energy W of the scintillation lies within a predetermined energy range.
This detection chain is followed by a visual display array, commonly comprising a cathode-ray oscilloscope controlled by the X,Y coordinates and the signal Z, for visually displaying the point of impact of the gamma photon on the crystal by means of a luminous dot on the screen. The visual display array may possibly comprise a photographic device so that an image of the organ observed may be produced in this manner by integrating a large number of luminous dots formed on the cathode-ray screen. It may moreover comprise a device for digital processing of the images.
Amongst other qualities a gamma camera should have is a satisfactory spatial resolution, that is to say the capacity to differentiate between closely spaced small radioactive sources, a satisfactory response regarding counting rate, that is to say the capacity to process a large number of events per unit of time, and an image quality unaffected by the energy of the isotope in question. The spatial resolution depends on the accuracy of calculation of the X and Y coordinates. The quality of calculation of these coordinates depends substantially on the physical laws governing the operation of the different parts of the gamma camera. Thus, the interaction between a gamma photon and the crystal gives rise to a luminous scintillation of which the intensity decreases exponentially with time. The time constant of this reduction is characteristic of the scintillator crystal utilised, being of the order of 250 nanoseconds for a thallium-activated sodium iodide crystal NaI(T1). This scintillation is detected simultaneously by several photomultiplier tubes. The luminous photons forming this scintillation release photoelectrons from the photocathodes of the photomultiplier tubes. The number of photoelectrons released in this manner fulfils POISSON's statistical law for a given scintillation. This means that the electrical contribution of a photomultiplier tube receiving a scintillation has an amplitude of a value which follows a POISSON statistical distribution and of which the mean value is a function of the energy of the incident luminous photons.
Since the scintillation is detected simultaneously by several photomultiplier tubes, the determination of the location of this scintillation on the crystal, which itself represents the seat of emission of the energising gamma photon, is obtained by calculating the position of the barycentre of the electrical contributions supplied by the assembly of photomultiplier tubes energised by this scintillation. According to ANGER, this calculation is performed in simple manner by injecting these contributions through a set of matrices of resistors. The values of these resistors are a function of the positions of the photomultiplier tubes to which they are connected. The positions of these tubes are located with respect to Cartesian reference axes of which the point of intersection is commonly situated at the centre of the network of tubes.
For a given scintillation, the most difficult problem to resolve consists in the determination with optimum precision of the mean value of the amplitudes of each of the contributions. It is known that these contributions may be integrated in time over a period of the order of the decay time constant of the scintillations of the scintillator crystal. The period of this integration is typically of the order of three times the time constant. The period of integration or counting is a direct derivation from the POISSON statistic. As a matter of fact, the typical variation of the fluctuation of the amplitude of the contributions according to POISSON's statistic is inversely proportional to the square root of the number of photo-electrons released. Consequently, the longer the integration, the greater will be the number of photoelectrons taken into account, and the smaller will be the typical variation, and the greater will be the precision with which the mean value of this contribution is determined.
As a matter of fact, the operation for calculating the location of the barycentre being a linear operation, it is more economical to perform this integration at the output of each of the matrices of resistors of the array of matrices. In effect, these matrices merely perform a weighting or balancing operation on these contributions as a function of the location of the tubes on the crystal. The electrical pulses supplied at the output of the array of matrices of resistors are referred to as weighted or balanced pulses. It will be noted in passing that the counting period is related directly to the quality of spatial resolution of the gamma camera, but that this quality is obtained at the expense of the counting rate, that is to say at the expense of the number of events per second taken into account.
This integrating operation cannot be performed without some difficulties. The principal difficulty consists in the presence of constant direct voltages which are superimposed over the balanced pulses supplied by the matrices and which upon being fed into integrators, falsify the value of the signal supplied by these in direct proportion with the length of the integration period. The origin of these direct voltages consists principally in the fact that variable gain amplifiers are interposed between each matrix of resistors and a corresponding integrator. These variable gain amplifiers are utilized for two reasons: firstly, they serve the purpose of selecting the energy range which is to be examined, and secondly, they perform a matching of the amplitude of the balanced pulses to the operational dynamics of the integrators utilized. It will be observed that these direct voltages which are to be eliminated may have different origins, in particular that resulting from an action referred to as a clutter of scintillations.
The electrical potential applied by these direct voltages alters what is commonly referred to as the base potential of the integrators. U.S. Pat. No. 3,984,689 of Roger E. Arsenaux, issued on 5th of Oct. 1976, disclosed that at high levels of radioactivity, that is to say at high counting rates which for example exceed 100,000 events per second, capacitive couplings which may have been considered first of all for elimination of these direct voltages, should be prescribed. In effect, the presence of such coupling capacities causes an alteration of the base potential linked essentially with the very rapid repeated appearance of the scintillations. These capacities result in an action restoring a D.C. component which depends on precisely the counting rate. Nevertheless, the levels of precision required at present in calculating the coordinate signals, impose the need for the erratic amplitude changes of these signals to remain smaller than a thousandth of their amplitude. Although the solution proposed in this U.S. Patent is the best in theory, it is inapplicable in practice because of the uncontrollable drift of the direct voltages of the various elements of the electronic chain, as the authors of the present invention were able to observe.